[Mpi-forum] another request for iaccept

Jeff Hammond jeff.science at gmail.com
Thu Feb 25 23:55:50 CST 2016

On Thu, Feb 25, 2016 at 3:40 PM, Balaji, Pavan <balaji at anl.gov> wrote:

> > On Feb 25, 2016, at 2:48 PM, Jeff Squyres (jsquyres) <jsquyres at cisco.com>
> wrote:
> >> 1) Low overhead (justifies Isend/Irecv etc.)
> >> 2) Scarcity of threads (e.g., the BlueGene/L rationale)
> >
> > Agreed -- neither of these are likely important for an iconnect/iaccept
> scenario.
> I would disagree.  This is *always* a problem since adding threads hurts
> other operations in the application.  For example, if I need to use a
> nonblocking iconnect/iaccept in one small part of the application, it now
> means that every fine-grained PUT/GET/ISEND operation in the rest of the
> application would be more expensive.
Does making a blocking operation nonblocking via threads not require
MPI_THREAD_MULTIPLE, which Open-MPI does not support (
https://github.com/open-mpi/ompi/issues/157) more than 12 years after its
standardization?  It seems that a significant fraction of the MPI Forum
does not believe that thread-safe MPI calls are important, so how can
anyone argue that threads are a solution to this problem?


> > But I do think the progression overlap with application threads can be
> quite useful.
> Right.  Having a nonblocking operation is not about performance
> improvements, but that I can now stick in a request into an existing
> Waitall or Testany in my application.  FWIW, at least one of our
> applications uses NBC I/O in exactly this way.  Before MPI-3.1, they had to
> do an event-based model (with Testany) for everything else and a blocking
> call for I/O, which was inconvenient and hurts performance.
> >> There are some interactions with multiple-competion routines and
> limitations in the generalized requests, but fixing generalized requests
> would be a more general solution.
> >
> > Agreed -- fixing generalized requests has been a white whale for quite a
> while now.
> There are technical reasons for why this was not easily fixed, unlike
> iconnect/iaccept where people are bandwidth limited to put together a
> proposal.
>   -- Pavan
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Jeff Hammond
jeff.science at gmail.com
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